With an increased speed and capacity of communication, introduction of ultra high-speed wavelength division multiplexing (WDM) optical transmission systems with the communication speed of 10 Gbps, or even exceeding 40 Gbps, is in progress. In such high-speed optical transmission systems, an optical transmission signal waveform in an optical fiber transmission line is deteriorated by a wavelength dispersion characteristic of the transmission line, causing deterioration of the transmission quality. Accordingly, compensation of the wavelength dispersion is required.
As a means for compensating the wavelength dispersion in the optical fiber transmission line, a dispersion compensation fiber (DCF) has been known.
FIG. 1 shows a diagram illustrating the relation between the wavelength dispersion characteristic of an optical fiber transmission line and the wavelength dispersion compensation using the DCF conventionally in use. The vertical axis shows the wavelength dispersion magnitude, while the horizontal axis shows the wavelength of the signal light. The dispersion value (a) of the optical transmission line has a positive value that increases substantially linearly with the increase of the wavelength, and also a positive dispersion gradient (dispersion slope). Therefore, if the wavelength dispersion is compensated by the dispersion value (a′) having an ideal slope being symmetrical to the dispersion value (a) referenced from the wavelength dispersion of 0 ps/nm, the wavelength dispersion will be compensated completely. The DCF having been in use conventionally has the dispersion value (b) and the slope of which sign is inverted (minus) from the dispersion value and the gradient (slope) of the optical fiber transmission line. However, the slope is smaller than the ideal slope of the dispersion value (a′). Accordingly, when using the conventional DCF, it is not completely possible to compensate the wavelength dispersion produced in the optical fiber transmission line. Namely, even if the wavelength dispersion of the optical fiber transmission line is compensated by the DCF, the difference (c) caused by the residual dispersion wavelengths is produced.
FIG. 2 shows a schematic configuration diagram of the wavelength division multiplexing optical transmission system. As shown in this FIG. 2, in a transmission unit 10, a plurality of optical signals each having different wavelength fed from a plurality of optical transmitters 12 are multiplexed by a multiplexer 14, which are further amplified by a non-illustrated optical amplifier and output to an optical fiber transmission line 20 as wavelength multiplexed signals. In the middle of optical fiber transmission line 20, optical repeaters 22 are disposed at predetermined distance intervals, and the attenuated wavelength-multiplexed signal are amplified without being converted from the optical signal. Further, in each optical repeater 22, a dispersion compensation fiber (DCF) 24 having the wavelength dispersion compensation characteristic shown in FIG. 1 is inserted, by which the wavelength dispersion produced in optical fiber transmission line 20 of each predetermined distance interval is compensated collectively over the whole wavelength bandwidth of the wavelength-multiplexed signal.
However, as described earlier, residual dispersion is produced even after the dispersion is compensated using DCF 24. When the optical fiber transmission line has a long distance, undesirably the dispersion exceeds a tolerable dispersion value because of the accumulated residual dispersion. To cope with this problem, in the middle points of optical fiber transmission line 20, compensation nodes 30 are provided at intervals of, for example, a few hundred kilometers, for the purpose of compensating the accumulated residual dispersion. More specifically, each compensation node 30 includes a wavelength dispersion compensation unit 32 for compensating the accumulated residual dispersion, and further wavelength dispersion compensation unit 32 includes a DCF. Wavelength dispersion compensation unit 32 in compensation node 30 compensates the wavelength dispersion, which is accumulated while the wavelength-multiplexed signal is transmitted through optical fiber transmission line 20. Compensation nodes 30 are implemented at necessary intervals, so that the residual dispersion value produced after the above compensation may not exceed a tolerable dispersion value (dispersion tolerance). Thus, after relayed by compensation nodes 30, the wavelength-multiplexed signal reaches a reception unit 40. A demultiplexer 42 in reception unit 40 demultiplexes the signal into optical signals of each wavelength. Then receivers 44 receive the demultiplexed optical signals of each wavelength under tolerable wavelength dispersion conditions.
Now, as shown in FIG. 1, since the wavelength dispersion magnitude differs for each wavelength, when compensating an input wavelength-multiplexed signal collectively by the DCF provided in wavelength dispersion compensation unit 32, if the wavelength dispersion compensation is performed to meet the wavelength dispersion magnitude on the short wavelength region, the compensation magnitude in the long wavelength bandwidth becomes insufficient, and the wavelength dispersion exceeds the dispersion tolerance. On the other hand, if the wavelength dispersion compensation is performed to meet the wavelength dispersion magnitude on the long wavelength region, the compensation magnitude in the short wavelength bandwidth becomes excessively large, which undesirably exceeds the tolerable dispersion value on the inverse sign side.
Accordingly, in wavelength dispersion compensation unit 32, using DCFs corresponding to the compensation magnitude required for each channel signal of different wavelength after wavelength-dividing the wavelength-multiplexed signal, it is necessary to adjust the residual dispersion magnitude of each channel signal to the vicinity of 0 ps/nm, and thereafter multiplex the signals again and forward the signal to the optical fiber transmission line.
However, to apply different DCFs for each wavelength makes the configuration of wavelength dispersion compensation unit 32 complicate, which produces an increased cost. Therefore, to cope with this problem, there is a known method of wavelength grouping, in which the wavelength bandwidths of the wavelength-multiplexed signal are divided into a plurality of groups, and wavelength dispersion compensation is performed collectively on a group-by-group basis, for the groups that include a plurality of channel signals.
FIGS. 3A shows an example of wavelength dispersion compensation using the wavelength grouping method. As shown in this FIG. 3A, a wavelength-multiplexed signal is divided into six groups, and compensation magnitude is adjusted for each group so that the residual dispersion magnitude falls within a tolerable dispersion value. FIG. 3B shows a configuration example of wavelength dispersion compensation unit 32 when the wavelength grouping method corresponding to the example shown in FIG. 3A is applied. An input wavelength-multiplexed signal is demultiplexed into six groups by a demultiplexer 322. For the group (1) positioned on the shortest wavelength side, no wavelength dispersion compensation is performed by the DCF because the wavelength dispersion magnitude does not exceed the tolerable dispersion magnitude. For the groups (2) to (6) because a group having longer wavelength bandwidths produces larger wavelength dispersion magnitude, DCF 324 is applied for each group, so that larger compensation magnitude is acquired in accordance with the produced wavelength dispersion magnitude. Each channel signal is multiplexed again in multiplexer 328 via each variable optical attenuator (VAT) 326, and then forwarded as a wavelength-multiplexed signal. Here, each VAT 326 is provided for adjusting transmission loss differences between each wavelength bandwidth, in case of necessity.